How to scale two UV islands of different sizes to be an identical size?
If you just have 2 islands and you don't care how they are oriented, just choose the Normalize option in the UV's menu. That is what it's for.

But let's say you took your time manually unwrapping and arranging several islands and need to match the scale of both groups but don't want to ruin the work you already did.


select only the relevant faces in the model (faces you need to
normalize). Make sure the option in the upper-left to link the model
select to the UV layout is OFF.
You will see in the UV editor that only the selected faces are
visible. This also means that operations in the UV editor will not affect hidden UVs.
Choose from the menu in the UV editor to Normalize the UVs. Now all of those UVs are the same size. This WILL reposition and CAN (unless you uncheck the option) rotate the islands though, so if that is a problem you will need to rotate them back into position now.
Drag those faces over someplace where they will be together, but not intersect other UVs. Select all the faces on your model. You will see that all UVs are visible in the UV editor now. Manually scale the islands you just normalized to make sure all islands are perceptually the same size.


This way, you don't have to touch islands that are already the right size and in the right position.